After several attempts ending in failure I've been forced to conclude that [Jajah][2], the internet based VoIP client, doesn't work with the iPhone. What's worse is, even using Firefox from the desktop, I can't get Jajah to work with the iPhone. 

The call goes through and I can hear the other caller quite clearly (despite the usual VoIP lag-time and echos), but no one seems to be able to hear me from the iPhone.

Obviously Skype isn't an option, though hackers claim to be quite close to discovering a way to [install third party software][1], which leaves the iPhone without a VoIP option.

If anyone out there has any suggestions for a web-based VoIP client I should test, be sure to leave a comment below.

[1]: http://www.hackszine.com/blog/archive/2007/07/all_about_iphoneinterface.html
[2]: http://jajah.com/